In recent years, a cyclic olefin-based copolymer obtained by copolymerizing ethylene and a specific cyclic olefin is attracting attention as a plastic-based optical material requiring transparency. The cyclic olefin-based copolymer is excellent not only in the transparency but also in the resistance against moisture, chemicals, heat and the like and its application is expected to expand, for example, into an optical material such as optical fiber and plastic resin (see, for example, JP-A-6-93040 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and Japanese Patent 2,619,856). Studies are also being made on use as an optical film and with regard to a film capable of improving the hygroscopic property and moisture permeability of the conventionally employed cellulose acetate film, development of a polarizing plate protective film produced by heat melt film formation or solution film formation is proceeding (see, for example, JP-A-2002-114827 and JP-A-2007-9010).
However, the copolymer composed of ethylene and a cyclic olefin is hard and brittle and has a problem that the mechanical strength is low and the handleability and processability are bad.
Also, a copolymer of an α-olefin having a carbon number of 4 to 12 and a cyclic olefin is reported (see, for example, Polyhedron, Vol. 24, pp. 1269-1273 (2005) and JP-A-2007-119660). However, the copolymer described in Polyhedron, Vol. 24, pp. 1269-1273 (2005) has a small molecular weight and is not suitable as a molding material. The copolymer obtained by the production method described in JP-A-2007-119660 has an appropriate molecular weight and can be improved in the brittleness as compared with the copolymer composed of ethylene and a cyclic olefin while maintaining the properties such as transparency and moisture resistance.